The present invention relates to directional microphone systems.
For improved pickup of sounds in the presence of ambient noise, directional microphones are quite advantageous. Directional microphones that achieve low frequency directionality are especially useful since most interfering noise energy is located at low frequencies
In hearing aids, directional microphone technology can result in significant noise reduction. Typically, in hearing aid systems, the desired signal comes from the front of the user while noise tends to be ambient including a large component from the rear. In the communications field, it is important to reject noise sounds that occur in the band between 300 Hz and 1000 Hz (1 KHz). In both hearing aid and communication systems, directionality, especially low-frequency directionality, directly converts into better product efficiency.
FIGS. 1A and 1B illustrate an omnidirectional (zeroth order) microphone. An omnidirectional microphone is equally sensitive to sounds arriving from any direction. A common measure of microphone directionality is the ratio of on-axis sensitivity to the integral of sensitivity to sounds arriving from all angles. This measure is called the directionality index (DI), often expressed in decibels. An omnidirectional microphone has a DI of 1, or 0 dB.
FIGS. 2A–2F illustrates a first-order microphone. Miniature first-order microphones can be created with two omni-directional elements and an electrical circuit, as shown in FIG. 2A. Alternatively, the first-order microphone can be created with a single pressure-gradient element using an acoustic circuit, shown in FIG. 2B, instead of the electrical circuit. FIG. 2A shows two omnidirectional microphones 20 and 22, separated by a propagation distance of τp. The output of one of the omnidirectional microphones, omnidirectional microphone 22, is sent to a delay line 24. The output of the delay line 24 is subtracted from the output of the omnidirectional microphone 20 with combiner 26. FIG. 2B shows an acoustical first-order microphone unit. The first-order pressure-gradient element 30 includes a front sound inlet port 32, a rear sound inlet port 34, and a diaphragm 36. An acoustical delay line 38 is used to acoustically delay sound coming from one of the inlet ports. Since the sound impinges upon both sides of the diaphragm 36, pressure on one side is effectively subtracted from the pressure on the other side.
Classically, several of the first-order directionality patterns have been found to be useful and have been given names. Each pattern is produced when the internal delay, electrical or acoustic, τd, equals a specific fraction of the free field propagation delay, τp, for the incident sound wave to propagate from one sound inlet port to the other. For example, if the internal delay is adjusted to equal the propagation delay, the delay ratio, DR=τd/τp, is equal to 1, and the directionality pattern is the well-known cardioid pattern shown in FIG. 2F. The cardioid has a single null directly to the rear and a DI of 4.8 dB. Another classical directionality pattern is the hypercardioid, created when DR equals one-third. This pattern, shown in FIG. 2D, has two nulls, a moderate backlobe, and exhibits the best directionality index (DI=6 dB) of the first-order elements. For better rejection of sound from the rear, the supercardioid pattern is used. This pattern is created when the internal delay is set to 1/√{square root over (3)} times the propagation delay, DR=approximately 0.58 and DI=5.7 dB. This example is shown in FIG. 2E. Another classical pattern of importance is the “figure eight” or bipolar pattern shown in FIG. 2C, which is used when low sensitivity to sounds from the sides is desired. This pattern has a DR=0 (no internal delay) and a directionality index of 4.8 dB.
All the first-order free field directionality patterns can be described with the equation
      A    ⁡          (      θ      )        =            1      +                        (                      1            /            DR                    )                ⁢        cos        ⁢                                  ⁢        θ                    1      +              (                  1          /          DR                )            where θ is the angle of sound arrival relative to the forward element axis, and DR is the delay ratio. Note that as DR goes to infinity (τd becomes infinite), the zeroth-order omnidirectional microphone is produced.
To produce a second-order (or higher-order) microphone, two or more omni- or first-order gradient microphones are combined, with an electrical delay circuit, or with an acoustic circuit, to create an end fire directional array. In any case, the array can be considered to be a combination of first-order gradient microphone units, whether developed from omni- or pressure-gradient elements. FIG. 3A illustrates an example of a second-order microphone system constructed of two bipolar first-order microphone elements adapted from the article by Olsen, “Directional Microphones,” pp. 190–194, of An anthology of articles on microphones from the pages of the Audio Engineering Society, Vol. 1–Vol. 27 (1953–1979). This second-order microphone system has a very high directionality pattern as shown in FIG. 3B. A theoretical directionality index of 9.0 dB is produced by this method.
Second-order microphone arrays designed using first-order microphone elements give excellent theoretical directionality patterns. Unfortunately, such second-order microphone systems have been unsuccessful when used on the side of the head, for example in a hearing aid. In all such previous second-order microphone array systems used at the side of the head, the performance of the microphone array in situ degrades to below that of a first-order microphone element, such that there is no benefit to the second-order configuration. The near-field diffraction effects that result from placing the second-order microphone next to the user's head degrade the system performance. These near-field diffraction effects cannot be adequately compensated for, especially where a single microphone design is intended for use by numerous individuals each with their own unique head shape and size, i.e. biological variability.
It is desired to have an improved microphone system for use on the side of a user's head.